I didn't notice a difference between mobile 5w30 (conventional, not m1) and the T6 when I switched. Start-up was the same. Even at colder temps, like recently the mornings were down to below 30*, it took maybe a 1/2 second longer to crank over.

Not this oil change but my next one, I will get the T6 tested and see how it's been doing.

That's because 30*F is not very cold. Try that 60* colder (-30*F).

I've used RT6 in my WRX through a MN winter, but the coldest I ever had to start it unaided was about 0*F. I've also used GC and Redline 0w30 in the winter, believe me, there's a substantial difference.

There are several phenomena which conspire to make starting much harder at cold temperatures. The battery looses it's ability to provide current, the oil gets thicker, requiring more power from the starter to spin the engine at a given RPM, and the fuel becomes harder to vaporize, meaning you need to spin the engine faster to generate the port velocity required for a viable combustion charge. Add it all up and the difference between 30*F and 0*F is a lot greater than the difference between 60*F and 30*F.

Here's what the relative viscosities of M1 oils and RT6 look like down to 32*F (0*C). RT6 is over 50% thicker at 32*F than M1 5w30.